Heating boilers for heating the rooms of a building and/or for providing tap hot-water are usually controlled by a thermal relay which provides an on/off temperature control. Such boilers are typically dimensioned in such a way that they are able to manage conditions, in which an extremely high heating demand exists, i.e. during the coldest winter days. This means that the capacity of the heating boiler is utilised to the greatest extent during the cold periods of the year. During the remaining parts of the year, the boilers are over-dimensioned, and consequently, the overall efficiency is not optimal.